1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
International Journal of Electronics and Communication
IJECET
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
Engineering & Technology (IJECET)
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online)
Volume 2, Number 1, Jan – April (2011), pp. 57-62 ©IAEME
© IAEME, http://www.iaeme.com/ijecet.html
COMPOSITIONAL AND ELECTRICAL PROPERTIES OF CDS
THIN FILMS BY VACUUM EVAPORATION METHOD
D KATHIRVEL a*, N SURIYANARAYANAN b, S PRABAHAR c, S SRIKANTH c
a* Department of Physics, Kalaignar Karunanidhi Institute of Technology, Coimbatore,
India
b Department of Physics, Government College of Technology, Coimbatore, India.
c Department of Physics, Tamilnadu College of Engineering, Coimbatore, India
ABSTRACT
Cadmium Sulphide thin films have been deposited on to well cleaned glass
substrate in a vacuum of 10-6 Torr. The thickness of the films has been determined by
quartz crystal monitor method. The compositional analysis is carried out by Energy
dispersive X-ray analysis (EDAX). The electrical resistivity measurements were
performed at room temperature by four probe method and it shows CdS films with high
resistivity in the order of 10-6 cm. Thermal activation energy is also calculated by varying
the thickness of the films and no systematic variation of activation energy is observed.
Keywords: Vacuum deposition CdS thin film, EDAX analysis, Electrical properties.
1. INTRODUCTION
The necessity and interest in the analysis of thin films have been greatly
stimulated in recent years because it has been the subject of countless investigation [1-3].
CdS thin films have gained much attention during the past two decades due to their
spectral properties such as high refractive index [4-6]. CdS is a suitable window layer for
solar cells and also finds application as optical filters and multilayer light emitting diodes,
photo detectors, thin film field effect transistors, gas sensors and transparent conducting
semiconductor [7-8]. Semiconducting thin films have been extensively studied for a long
time due to their significant role in modern science and technology. Among various
methods the reliable, simple and cost effective route to synthesis CdS thin films in
vacuum evaporation technique. This method is the most widely used technique for the
deposition of metals, alloys and also many compounds. This involves the evaporation for
sublimation of the material in vacuum by thermal energy and allowing the vapor stream
of the charge to condense on a substrate so as to form continues and adherent deposit of
desired thickness [9-10]. The objective of the paper is to study the compositional and
electrical properties of CdS thin films.
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2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
2. EXPERIMENTAL DETAILS
The CdS powder of purity 99% was evaporated using Tungsten conical basket
(200 amps) under the pressure of 2 x 10-5 Torr on to a pre cleaned glass substrate (3.25 x
2.75 x 0.1 cm dimension). The pressure was obtained by diffusion pump backed by rotary
pump in the coating unit and was measured using Pirani and Penning gauge. A constant
rate of evaporation of the order of 1 Å / sec was maintained throughout the film
fabrication. A rotary device was employed to maintain uniformity in film thickness. The
thickness of the film was controlled and measured by Quartz crystal monitor and the
thickness monitor in a flat circular plate approximately 0.05 inch (1.4cm) in diameter and
0.011 inch (0.28 cm) thick. A substrate heater arrangement was employed to grow the
thin film at different substrate temperature. The Copper – constant and thermocouple was
employed to measure the temperature inside the champer. The specimen is mounted in
the centre of the diffractometer and rotated by an angle around an axis in the film plane.
Energy dispersive X-ray analyzer (LEICA.S44Oi) confirmed the composition of the
constituents in CdS thin films. The most commonly used technique in the semiconductor
industry for measuring resistivity is the four point probe method. The arrangement
consists of PID controlled over (Model PID – 200, Scientific Equipment and services,
Rookee, India) is combined to low current source, constant current source (Model LCS -
01) and digital micro voltmeter (Model DMV-001).
3. RESULT AND DISCUSSION
3.1. EDAX ANALYSIS OF CDS THIN FILMS
Fig. 1, Fig. 2, & Fig. 3, shows the energy dispersive X-ray spectrum of CdS thin
films having thickness of 880 Å, 930 Å & 2550 Å.
Fig. 1. EDAX spectrum of CdS thin film of thickness 880 Å
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3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
Fig. 2. EDAX spectrum of CdS thin film of thickness 930 Å
Fig. 3. EDAX spectrum of CdS thin film of thickness 2550 Å
From the table 1, the elemental composition of Cadmium, Sulphide and Oxygen is
found to be in various thicknesses of 880 Å, 930 Å & 2550 Å. The of oxygen due to the
using of octylamine. The concentration is observed to be varying with film thickness, but
systematic variation is not observed.
Table. 1. Elemental composition of CdS thin films.
Wt % At %
Element
880 Å 930 Å 2550 Å 880 Å 930 Å 2550 Å
Cd L 67.15 67.73 67.73 16.12 20.71 20.71
SK 07.64 15.57 15.57 04.85 12.61 12.61
OK 25.21 16.70 16.70 79.03 66.68 66.68
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4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
3.2. ELECTRICAL PROPERTIES
The electrical resistivity of CdS films with different thickness was measured
using the d.c. four probe method in air. Fig. 4. Fig. 5 & Fig. 6, shows the variation of
resistivity ( cm) with reciprocal of temperature (1000/T). For all films, it was seen that
resistivity decreases with temperature indicating semiconducting nature of films [11-14].
For all the films, resistivity follows the relation,
ρ = ρ0 exp (E0 /KT) -------------------- (1)
Where ‘ρ’ is resistivity at temperature ‘T’, ρ0 is a constant, ‘K’ the Boltzmann
constant (1.38 x 10-23 J/k) and ‘E0’ the activation energy required for conduction.
Resistivity of CdS thin film decreases due to the improvement in crystallinity of the films
as the film thickness increased. This observation is attributed to the size effect observed
in semiconductor thin films.
Fig. 4. Resistivity Vs (1000/T) of CdS thin film of thickness 880 Å.
Fig. 5. Resistivity Vs (1000/T) of CdS thin film of thickness 930 Å.
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5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
Fig. 6. Resistivity Vs (1000/T) of CdS thin film of thickness 2550 Å.
From the resistivity plot, the thermal activation energies were calculated using the
formula (1). Table 2. Show the activation energies at different thickness of CdS film.
Activation energies are to the order of 0.6345 to 0.7095 eV as film thickness was
increased from 880 Å to 2550 Å.
Table 2. Variation of activation energy with film thickness.
Film thickness (Å) Activation energy (eV)
880 Å 0.6345
930 Å 0.6547
2550 Å 0.7095
4. CONCLUSIONS
The CdS thin films have been deposited by vacuum evaporation and
thickness of film was varied from 880 Å to 2550 Å, changing deposition time. Energy
dispersive x-ray analyzer confirms the composition of constituent in the CdS thin films.
The concentration is observed to be varying with film thickness, but systematic variation
is not observed. The electrical resistivity measurements were performed at room
temperature by four probe method and it shows CdS films with high resistivity. The
activation energy is increases with increasing in film thickness.
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6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976
– 6464(Print), ISSN 0976 – 6472(Online) Volume 2, Number 1, Jan - April (2011), © IAEME
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